This invention relates to a trackerball arrangement in which rotation of a ball by hand movement causes the generation of corresponding electrical signals. Such arrangements are commonly used in conjunction with display screens for the purpose of moving a cursor or the like around the screen. The electrical signals may be indicative of the extent of rotation of the ball or merely of its sense. It is customary for separate electrical signals to be generated which are respectively indicative of rotation about orthogonal x and y axes, and these signals may be derived from the rotational motion of the rollers which bear against the surface of the ball. As the ball has a smooth surface, the frictional grip between the ball and a roller is very low, and any resistance by a roller to rotation will result in the roller slipping against the surface of the ball so that the resulting electrical signals do not faithfully represent the movement of the ball.
Any imperfection in the operation of the bearings, by which a roller is supported, degrades the operation of the trackerball arrangement. It has been found that the bearings and rollers are susceptible to shocks delivered via the ball.
A trackerball arrangement which reduces the effects of shock is described and claimed in British Patent Specification No. 2221016B in which a ball is rotatably mounted with respect to a housing; and a plurality of supports are arranged to locate the ball in a predetermined operable position, at least one of the supports being mounted for movement away from the other supports; and said one of the supports being urged towards said other supports against a stop by the action of resilient means acting upon it; whereby pressure which is applied via the ball to the supports causes movements of said one of the supports when the applied pressure exceeds a threshold value which overcomes the force exerted by said resilient means, said movement allowing lateral movement of the ball away from the other supports. In this way excessive shocks applied to the top of the ball do not cause damage to the ball supports at the point of contact between them.
The lateral movement of the ball away from the other supports is arranged preferably to allow the ball to drop, such that shock impacts on the ball are largely absorbed by the downwards movement of the ball and the resilience of a support.